He didnt use a scope due to reflection giving away his positions, and he would pack the snow in front of him to help stop any muzzle puff when he fired.
He also killed nearly all of the squads sent out to kill him.
He was apparently a very humble man, and his discipline when sniping was second to none.
edit: also, to take 500 men in 100 days is incredible, but to add 200 more that total with his secondary is almost unheard of.
The kills he has been credited for, dont count unconfirmed, which were probaly considerably more.